The invention relates generally to the field of power supplies. More specifically, the invention relates to a method and apparatus for accepting a wide range of input power and controlling the output power in a plasma arc power supply and control system.
Power supplies typically convert a power input to a necessary or desirable power output tailored for a specific application. In plasma arc cutting applications, power sources typically receive a high voltage alternating current (VAC) signal and provide a high current, plasma arc cutting output. Around the world, utility power supplies provide sinusoidal line voltages of 200/208V, 230/240V, 380/415V, 460/480V, 500V and 575V. The signals provided by these supplies can be either single-phase or three-phase and can have frequencies of either 50 or 60 Hz. Power supplies for plasma arc systems receive such inputs and produce a high current DC output of approximately 10-400 amperes for use as plasma arc cutting output.
In plasma arc cutting, high energy power is delivered to a plasma arc torch to produce an arc that generates heat sufficient to cut a metal workpiece. There are many types of power supplies capable of providing power sufficient for generating a plasma arc cutting applications. Some prior art power supplies are resonant converter power sources that deliver a sinusoidal output. Other power supplies provide a squarewave output. Yet another type of power supplies is an inverter-type power source.
Inverter-type power supplies are particularly well suited for plasma arc cutting applications. An inverter power supply can provide an ac square wave or a dc output. Inverter power supplies also provide for a relatively high frequency stage, which provides a fast response in the plasma arc current output to changes in the control signals.
In general, an inverter-type power source receives a sinusoidal line input, rectifies the sinusoidal line input to provide a dc bus, and inverts the dc bus and can rectify the inverted signal to provide a dc output. It is desirable to provide a generally flat and regulated, i.e. very little ripple, dc bus. Accordingly, it is not sufficient to simply rectify the sinusoidal input; rather, it is necessary to also smooth, and in many cases alter the voltage of, the input power. This is called preprocessing of the input power. The ability to automatically adapt to a number of input power voltage magnitudes is advantageous since different voltages can be used in plasma arc cutting.
To create the adaptability, the power supply requires several stages that require different size magnetics and different switching frequencies. To control the heat dissipation and the size of the magnetics, there is a need to control the adaptable power supply in an integrated fashion. With the several frequencies of the power supply, there is a need to integrate control of the power supply to increase allowable alternate design strategies. This invention addresses these needs.
This invention relates to methods and apparatus for controlling a power supply of a plasma arc system. In one embodiment, the power supply includes an input stage, a power factor corrected boost stage and an inverter stage. In one aspect, the invention relates to a method of controlling a power supply of a plasma arc system. According to the method, any AC input voltage within a range of input voltages is provided into the input stage and a rectified output voltage is thereby generated. The rectified output voltage is provided into the power factor corrected boost stage and a DC signal is thereby generated. The DC signal is provided into an auxiliary power supply and a regulated power signal is thereby generated. The regulated power signal is provided into a digital signal processor module and an output control signal is thereby generated. The output control signal is provided into the inverter stage and a plasma arc current is thereby generated.
In one embodiment, the method includes the step of generating the output control signal based on an average duty cycle of the inverter stage. In another embodiment, the method includes the step of providing into the digital signal processor module at least one of a start signal generated in response to a user input and a current command signal generated in response to a user input and a current feedback signal generated in response to current flow through a plasma torch electrode. The method can include the step of providing the output control signal to the inverter stage based on the regulated power signal and at leas one of the start signal, the current command signal and the current feedback signal.
In another embodiment, the method includes the step of providing into the digital signal processor module a rectified voltage signal generated in response to the rectified output voltage of the input stage. The method can include the step of limiting the plasma current or voltage based on the rectified voltage signal. In another embodiment, the method includes the steps of providing into the digital signal processor module a transfer signal generated in response to current flow through a work piece and providing from the digital signal processor module to a nozzle in a plasma torch electrode a pilot arc control signal. The method can include the step of providing the output control signal to the inverter stage based on the transfer signal. The method can include the step of inhibiting current flow through the nozzle in the plasma torch electrode in response to the pilot arc control signal. The method can include the step of providing a relay control signal from the digital signal processor module to a relay that opens or closes a substantially short circuit in parallel with an inrush current limiting resistor provided in the power supply. The method can include the step of providing the relay control signal based on at least one of the regulated power signal and the rectified voltage signal.
In another embodiment, the method includes the step of providing to the power factor corrected boost stage from the digital signal processor module a second output control signal. The second output control signal can be based on the frequency of the switching of the inverter stage. The method can include the step of providing into the digital signal processor module a safety status signal generated in response to the removal of a retaining cap of a plasma torch electrode. The method can include the step of latching the state of the safety status signal when the safety status signal changes from a first value to a second value. The method can include the step of inhibiting the flow of the plasma arc current when the safety status signal is latched in a predetermined state.
In another embodiment, the method includes the step of using a harmonic injection algorithm to improve a power factor of the power supply. The method can include the steps of providing the DC signal to a transformer, providing from the transformer the regulated power control signal and switching the DC signal through the transformer in response to a value of the regulated power signal. The AC input voltage can be a single-phase signal or a multi-phase signal. In another embodiment, the method includes the step of generating a rectified voltage from the output of the inverter stage of the power supply. The output of the inverter stage is capable of producing the plasma arc current. The method can include the step of damping the generated rectified output voltage of the input stage. The method can include the step of filtering the AC input voltage prior to the step of providing the AC input voltage to the input stage.
In another aspect, the invention relates to a system for controlling a power supply of a plasma arc system. The system includes an input stage, a power factor corrected boost stage, an inverter stage, an auxiliary power supply and a digital signal processor module. The input stage includes a circuit for receiving any AC input voltage within a range of input voltages and generating a rectified output voltage. The power factor corrected boost stage is electrically connected to the input stage and includes a circuit for receiving the rectified output voltage and generating a DC signal. The inverter stage is electrically connected to the power factor corrected boost stage and includes a circuit for receiving the DC signal and generating an AC signal capable of providing a current to a plasma torch electrode. The auxiliary power supply is electrically connected to the power factor corrected boost stage and includes a circuit for receiving the DC signal and generating a regulated power signal. The digital signal processor module is in communication with the auxiliary power supply and the inverter stage and includes a circuit for receiving the regulated power signal and providing an output control signal to the inverter stage for generating a plasma arc current.
In one embodiment, the digital signal processor module is in electrical communication with at least one of a first switch and a second switch and a current sensor and the input of the power factor corrected boost stage. The digital signal processor module is configured to provide the output control signal based on the regulated power signal and at least one of the start signal and the current command signal and the current feedback signal and the rectified voltage signal. The first switch provides a start signal in response to a user input. The second switch provides a current command signal in response to a user input. The current sensor provides a current feedback signal in response to current flow through the plasma torch electrode. The input of the power factor corrected boost stage provides a rectified voltage signal in response to the rectified output voltage of the input stage.
In another embodiment, the system further comprises a pilot arc control switch in electrical communication with the digital signal processor module and a nozzle of the plasma torch electrode. The pilot arc control switch includes a circuit for inducing and inhibiting current flow through the nozzle in response to a pilot arc control signal generated by the digital signal processor module. In another embodiment, the system further comprises a relay in electrical communication with the digital signal processor module, the output of the input stage and the input of the power factor corrected boost stage. The relay opens or closes a substantially short circuit in parallel with an inrush current limiting resistor provided in the power supply in response to a relay control signal generated by the digital signal processor module. In another embodiment, the digital signal processor module is in electrical communication with the power factor corrected boost stage and is configured to provide a second output control signal to the power factor corrected boost stage. In another embodiment, the digital signal processor module is configured to provide the second output control signal based on the frequency of the switching of the inverter stage of the power supply. In another embodiment, the digital signal processor module is configured to provide the second output control signal based on a harmonic injection algorithm to modify the power factor of the power supply. In another embodiment the digital signal processor module is in electrical communication with a third switch. The third switch provides a safety status signal in response to the removal of a retaining cap of a plasma torch electrode. In another embodiment, the digital signal processor module further comprises a latching module for latching the state of the safety status signal when the safety status signal changes from a first value to a second value. The flow of the plasma arc current is inhibited when the safety signal is latched in a predetermined state. In another embodiment, the auxiliary power supply includes a flyback topology. In another embodiment, the input stage is configured to receive a single-phase AC input voltage. In another embodiment, the input stage is configured to receive a multi-phase AC input voltage. In another embodiment, the system further comprises a second rectifier stage electrically connected to the inverter stage and a plasma torch electrode. The second rectifier stage includes a circuit for generating a rectified voltage from the output of the inverter stage. In another embodiment, the system further comprises a damping stage electrically connected to the input stage and the power factor corrected boost stage. The damping stage includes a circuit for damping the rectified output voltage of the input stage. In another embodiment, the system further comprises a filtering stage electrically connected to the input stage. The filtering stage includes a circuit for filtering the AC input voltage prior to the input stage receiving the AC input voltage. In another embodiment, the system further comprises an on-board programming module in electrical communication with the digital signal processor module and an external storage device. The on-board programming module includes a circuit for transferring a computer program contained in the storage device to the digital signal processor module.
In another embodiment, the digital signal processor module further comprises an input conditioning module, an output conditioning module and a DSP device. The input conditioning circuit is configured to receive an input signal from source external to the digital signal processor module and to condition the input signal to a level acceptable to a digital signal processor. The output conditioning circuit is configured to receive an output signal from the digital signal processor and to condition the output signal to a level acceptable to a circuit external to the digital signal processor module receiving the conditioned output signal. The digital signal processor device is in electrical communication with the first conditioning circuit and the second conditioning circuit and is configured to receive the input signal and generate the output signal. In another embodiment, the digital signal processor device is configured to receive a computer program from an external device. In another embodiment, the digital signal processor device is configured to perform a self-health operation and to reset if the self-health operation is not executed properly. In another embodiment the second conditioning circuit is configured to receive the first output control signal which comprises a positive switch command signal and a negative switch command signal and to condition the output control signal to a level acceptable to the inverter stage. In another embodiment, the digital signal processor device is configured to generate the positive switch command signal and the negative switch command signal as a matched set with the same value at the same time frame of the signal generation.